The present invention relates to digital communications and more particularly, to method and apparatus for equalization and tracking of coded digital communications signals.
Decision feedback equalizer (DFE) and decision directed (DD) tracking loops are well known tools for equalization of digital communications signals and for adaptive compensation of dynamic channel impairments, phase noise, carrier frequency error, and dynamic impairments of the transmitter and the receiver. DFE and DD loops are based on a decision on the transmitted symbols. In the prior art, the decision rule is a memoryless xe2x80x9cnearest neighborxe2x80x9d rule (also called xe2x80x9cslicerxe2x80x9d) which yields the element in the set of transmitted symbols (i.e. the transmitted constellation) that has the smallest distance to the equalizer""s output data point or estimated symbol. However, when the signal is coded by a forward error correction (FEC) coder this slicer decision is less accurate than a decision which is taken by a decoder which exploits the FEC, i.e. uses the code to get better symbols. This is particularly a severe effect in systems that use trellis coded modulations (TCM) since TCM coders expand the constellation which effectively results in a decreased SNR. For example, the TCM coder of an Advanced Television System Committee (ATSC) digital TV (DTV) transmitter expands the constellation from 4-PAM to 8-PAM which increases the error probability of the slicer by a factor that is equivalent to a 6 dB decrease in SNR. The slicer errors propagate back through the DFE and increase the error at the equalizer output. This effect is particularly severe in situations where the DFE taps have large magnitudes, e.g. when there are reflections in the channel, or when the signal is contaminated by narrowband noise. Furthermore, the slicer""s errors cause tracking errors in the symbol clock and carrier recovery activities.
Long, Ling and Proakis (xe2x80x9cAdaptive transversal filters with delayed coefficients adaptationxe2x80x9d, Proc. Int. Conf. Acoustics, Speech, Signal Processing, Dallas, Tex., March, 1987) have proposed tracking the signal parameters in a DD manner, using a FEC decoder output. However, their approach adds decoding delay to the tracking loops which degrades the ability to track fast changes in phase noise, for example caused by changing reflections. The Long, Ling and Proakis approach is based on a linear equalization scheme, and it does not address the problem of error propagation in a DFE.
Eyuboglu and Qureshi (xe2x80x9cReduced-state sequence estimation for coded modulation on inter-symbol interference channelsxe2x80x9d, IEEE Journal on Selected Areas in Communications, August 1989) proposed an approach that jointly performs both equalization and TCM decoding. This approach, which is based on a reduced complexity approximation of the maximum likelihood sequence estimator (MLSE) Viterbi algorithm, may prevent degradation due to slicer errors, and it is aimed at achieving the performance of an MLSE based receiver, which is better than the performance of a DFE based receiver. However, the complexity of the Eyuboglu and Qureshi approach is much higher than the complexity of a DFE based receiver.
According to one embodiment of the present invention, a symbol detector in a decision feedback equalizer (DFE) loop is provided which exploits the structure of a forward error correction (FEC) encoder and outputs delay-less symbol estimates xc3xa20(t), where t is the time-index of the symbol. These estimates are fed back to a DFE loop and to a decision directed (DD) tracking loop. The symbol detector is also capable of generating delayed symbol estimates xc3xa2N(t), where N is the decision delay (where the higher delay estimates are likely to be more accurate than the low delay estimates), that are also fed back to the DFE and DD tracking loops. The DFE loop is capable of using xc3xa2N(t) as an input to its N,N+1,N+2, . . . taps. Should xc3xa2N(t)xe2x89xa0xc3xa2m(t), where N greater than M, the tracking loops are capable of correcting their parameters"" estimates accordingly, and the DFE loop is capable of repeating its last N-M iterations, using the more recent estimates. The outputs of the symbol detector may be a compromise (i.e. weighted average) between multiple constellation symbols, where the averaging weights depend on the estimated likelihood of each of these symbols.
The advantages of the proposed method and apparatus of the present invention are that the complexity of the present invention is similar to that of a conventional DFE receiver, but the error rate of the symbol detector is lower (and in some case, much lower) than those of a prior art slicer, yet no delay is added to the DD and DFE loops.
The method and apparatus of the present invention are particularly designed for equalization and phase tracking of digital terrestrial TV (DTV) signals transmitted according to the ATSC standard using vestigial side band (VSB) modulation, and digital signals transmitted over cable TV plants according to the DOCSIS spec using QAM modulation.
When applied to a VSB signal, the method of the present invention allows for combating severe NTSC co-channel-interference (CCI). Prior art methods for combating NTSC CCI degraded the noise threshold of the receiver by 3 dB (xe2x80x9cNTSC co-channel reduction systemxe2x80x9d U.S. Pat. No. 5,821,988, Citta et al., October 1998), while the method of the present invention allows for combating severe NTSC CCI without harming the noise threshold by more than a fraction of a dB.